Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session JM10: Mini-conference on Understanding Astrophysical Dynamos I |
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Sponsoring Units: GPAP Chair: Mark Nornberg, University of Wisconsin-Madison Room: 151 ABCG |
Tuesday, November 15, 2011 2:00PM - 2:25PM |
JM10.00001: The Origin and Evolution of Cosmic Magnetic Fields Ellen Zweibel Magnetic fields are observed in galaxies, clusters of galaxies, and possibly the intergalactic medium. In some cases, the fields display coherent structures on scales much larger than the turbulent injection scale. I will summarize the evidence for magnetic fields on cosmic scales in the contemporary Universe and in the distant past, and discuss current theories for their origin and evolution. [Preview Abstract] |
Tuesday, November 15, 2011 2:25PM - 2:50PM |
JM10.00002: Observational Constraints for the Galactic Dynamo in the Milky Way Jo-Anne Brown Magnetic fields are an important constituent of the interstellar medium, but unlike gas, dust and cosmic rays, they do not radiate, and consequently cannot be observed directly. Instead, observers identify {\it{signatures}} of the field in an effort to piece together its topology. Determining key parameters such as the number and location of magnetic field {\it{reversals}} (regions of magnetic shear where the field is observed to reverse directions by roughly 180$^\circ$ with radius), the pitch angle of the magnetic field, and its scale height, are critical in order to determine the most likely models of how the field originally formed and how it is evolving. In this talk, I will review which parameters of the field are approaching general consensus, and which remain highly contentious, and what this means (from an observational perspective) for the dynamo theory of the Galactic magnetic field. [Preview Abstract] |
Tuesday, November 15, 2011 2:50PM - 3:15PM |
JM10.00003: Progress in 21st Century Large Scale Dynamo Theory and Connections to Accretion Disks Eric Blackman Understanding the growth and non-linear saturation of magnetic fields on spatial or temporal variation scales larger than those of the underlying forcing is the subject of large scale dynamo (LSD) theory. Large scale fields are not only observed in stars and galaxies but also likely play a dynamical role in accretion disks. They are now commonly found in simulations of the latter. The existence of large scale patterns and the need for a practical modeling tool motivates semi-analytic mean field theories that distill the essential physics. But traditional 20th century textbook mean field theory has been challenged for its linearity, its lack of inclusion of small scale field growth, and inability to predict LSD saturation. I will discuss how 21st century mean field theories have led to substantial progress in overcoming these shortcomings and offer promise for future work. I will also argue that traditional accretion disk theory and LSD theory actually comprise two complementary mean field theories which should actually be combined into a single theory of accretion that includes both radial and vertical transport. [Preview Abstract] |
Tuesday, November 15, 2011 3:15PM - 3:40PM |
JM10.00004: High Omega Gain in High Shear Dynamo Flow with Low Turbulence Stirling Colgate The omega-phase of the liquid sodium alpha-omega dynamo experiment at NMIMT in cooperation with LANL has demonstrated a high toroidal field B$_\phi$ that is 8 times B$_r$ where B$_r$ is the radial component of an applied poloidal magnetic field. This enhanced toroidal field is produced by the rotational shear in stable Couette flow within liquid sodium at high Re $\sim $ 1.4 x 10$^{7}$ and magnetic Reynolds number Rm $\sim $ 120. A small turbulence in stable Taylor-Couette flow is caused by Ekman flow at the end walls, which causes an estimated turbulence energy fraction of (delta v/v)$^2$ $\sim $ 10$^{-3}$. This result compared to three highly turbulent flow measurements with an omega gain of $\sim $1.4 is interpreted as ``turbulence results primarily in the diffusion and dissipation of magnetic flux as compared to the possible creation of magnetic flux by dynamo action''. Large scale low turbulence, coherent flows as opposed to turbulent flows alone are then required to create the magnetic fields of the universe. [Preview Abstract] |
Tuesday, November 15, 2011 3:40PM - 4:05PM |
JM10.00005: Magneto-Rotational Turbulence and Dynamo Action Fausto Cattaneo The magneto-rotational instability (MRI) is commonly invoked as the primary source of turbulence in accretion disks. Although the instability requires a magnetic field to develop, in certain cases, the resulting turbulence can act as a dynamo and can regenerate the magnetic field needed to enable the instability. This self-consistent loop offers an attractive universal mechanism for magnetization of accretion disks. In this talk we examine the evidence, mostly numerical, supporting the idea that MR turbulence could be self-sustaining. In particular we shall discuss the effects of boundary conditions, stratification and dissipative processes on the type of dynamo action that is possible in a disk. [Preview Abstract] |
Tuesday, November 15, 2011 4:05PM - 4:17PM |
JM10.00006: Taylor-Couette dynamo Christophe Gissinger Dynamo action is a mechanism by which a magnetic field is self generated by the turbulent flow of an electrically conducting fluid. It is believed to be responsible for the magnetic field of many astrophysical objects. However, a lot of questions remain concerning the generation of fluid dynamos, in particular when the flow is strongly turbulent. On the other hand, Taylor-Couette flow is one of the most studied problem in fluids dynamics, with an abundant literature describing flow patterns, dynamical regimes, or the transition to turbulence as Re is increased. Surprisingly, only a few numerical work has been done on Taylor-Couette dynamo. I will present results of 3D direct numerical simulations of the magnetic field generated by a Taylor-Couette flow. I investigate the effect of turbulent fluctuations on the threshold of the dynamo and study the dynamical regimes obtained in the saturated state, depending on the flow patterns. Different configurations are explored, by varying the geometry of the cylinders, the rotation ratio and the magnetic boundary conditions. Finally, I describe the interaction between the dynamo magnetic field and the MagnetoRotational Instability (MRI) occurring in this simple Taylor-Couette configuration. [Preview Abstract] |
Tuesday, November 15, 2011 4:17PM - 4:29PM |
JM10.00007: Change of Magnetic Field Topologies in Plasma Disk Structures with Differential Rotation* Bruno Coppi Axisymmetric disk structures with a (toroidal) differential rotation balanced by a central gravity can be subject to the onset of collective modes that can produce new magnetic field topologies. Considering a currentless disk imbedded in a seed vertical field as a start, the driving factor of the relevant modes is the combination of a significant vertical temperature gradient and the differential rotation [1]. The modes can be axisymmetric producing periodic current channels, with alternate directions, or tridimensional consisting of radially localized trailing spirals. The two-dimensional magnetic field topologies expected from the non-linear evolution of these modes that are solutions of the relevant equilibrium equations are presented. *Sponsored in part by the U.S. DOE.\\[4pt] [1] B. Coppi, $\it{A\&A}$ $\bf{321}$, 504 (2009). [Preview Abstract] |
Tuesday, November 15, 2011 4:29PM - 4:41PM |
JM10.00008: Statistical Properties of Magnetic Fields in Galaxies Cameron Van Eck, Jo-Anne Brown, Andrew Fletcher, Anvar Shukurov Observations of magnetic fields in galaxies, including our own, provide critical constraints in modeling the origin and evolution of these magnetic fields, and how the fields are influenced by their physical environment. As a consequence of improved observational techniques, the number of galaxies with identifiable features and characteristics has increased significantly in recent years. There is now sufficient data to begin looking at statistical properties of galactic magnetic fields. In this talk, we present the results of correlation studies examining the relationship between both regular and random magnetic fields and other physical properties of galaxies, such as gas density and star formation rate. [Preview Abstract] |
Tuesday, November 15, 2011 4:41PM - 4:53PM |
JM10.00009: Small-scale Dynamo in the Intra-cluster Medium of Galaxy Clusters Hao Xu, Hui Li Radio observations suggest that magnetic fields of a few micro Gauss permeating the Intra-cluster medium (ICM) of galaxy clusters. Some observations and numerical simulations also suggest that the magnetized ICM is in turbulent state. The origin and evolution of the ICM magnetic fields are still not well understood, as well as the properties of the ICM MHD turbulence. Using adaptive mesh refinement (AMR) cosmological ideal magnetohydrodynamic (MHD) simulations, we study the magnetic field evolution and the MHD turbulence during the formation of galaxy clusters. We show that the cluster magnetic fields can be amplified by the turbulence through the small-scale dynamo process to the observation level. The weakly compressible ICM MHD turbulence is excited and maintained by the continuous mergers and accretion during the course of cluster formation. We will discuss the properties of magnetic fields and the MHD turbulence, as well as their relations to the operation of small-scale dynamo. The comparisons of simulated results with radio observations will also be presented. [Preview Abstract] |
Tuesday, November 15, 2011 4:53PM - 5:05PM |
JM10.00010: Magnetic Field Generation by Relativistic Shear Flows Edison Liang, Markus Boettcher, Ian Smith We report PIC simulation results of magnetic field generation by relativistic shear flows. We find that the shear flow boundary layer in initially non-magnetic shear flows is unstable to the growth of oblique 2-stream and Weibel instabilities near the boundary layer. Such instabilities generate current sheets and loops which eventually form nonlinear ordered structures resembling magnetic flux tubes with alternating polarity, orthogonal to the shear flow direction. Peak magnetic fields can reach almost equipartition values. The size and amplitude of such magnetic structures reach a steady state when the free energy input of the shear flow is balanced by turbulence dissipation. Nonthermal particles are efficiently accelerated, likely by the drift-kink instability, into a power-law energy distribution. These results have important implications for many astrophysical settings, including multi-component blazar jets and gamma-ray bursts. This work was supported by NSF AST0909167 and NASA Fermi grants. [Preview Abstract] |
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